1. Field of the Invention
This invention relates to an electric motor having a permanent magnet in a rotor, such as Brushless DC motor or the like and, more specifically, to an electric motor in which a magnetic flux density, a reluctance torque and so on can be selectively established, appropriate as a driving source of a compressor of an air conditioner, for example.
2. Description of the Related Art
In the electric motor of the type as described above, a permanent magnet is embedded in a core of an inner rotor of the electric motor, an example thereof being shown respectively in FIG. 8 and FIG. 9 each of which is a plane view showing the inside of this electric motor from a plan perpendicular to the axis of rotation.
In the example shown in FIG. 8 a rotor core 2 is disposed inside a stator core 1, having 24 slots for example, in which a field magnet rotates. In this case, the number of poles of the electric motor is four, therefore four permanent magnets 3 are arranged in the rotor core 2.
Each permanent magnet 3 is formed to be a band plate shape with rectangular cross-section, and each pair of the permanent magnets 3 is disposed to face each other along a direction perpendicular to a diameter line of the rotor core 2 in the vicinity of the outer circumference of the rotor core 2. Each permanent magnet 3 is embedded inside the rotor core 2 along a direction perpendicular to paper of FIG. 8.
Between the adjacent permanent magnets 3, holes 4 as flux barrier for preventing short-circuit and leak of magnetic flux between the adjacent permanent magnets are formed. In this example, the holes 4 are represented as triangle holes and disposed at both ends of the permanent magnet 3. In the center of the rotor core 2, a center hole 5 is formed to allow a rotating shaft (not shown) to pass therethrough.
In this structure, when the magnetic flux distribution in a gap portion (between teeth of the stator core 1 and the permanent magnet 3) caused by the permanent magnet 3 is in a sine wave state, torque T of the electric motor is expressed by, T=Pn{.PHI.a.multidot.Ia.multidot.cos .beta.-0.5(Ld-Lq).multidot.I2 sin 2.beta.}, where T is an output torque, .PHI.a is an armature flux linkage caused by the permanent magnet 3 on the coordinate axes d and q, Ld and Lq are the inductance on the axis d and the inductance on the axis q respectively, Ia is amplitude of an armature current on the coordinate axes d and q, .beta. is a lead angle of the armature current from the axis q on the coordinate axes d and q, and Pn is a pole logarithm.
In the above expression, the first term expresses a magnet torque 15 generated by the permanent magnet 3, and the second term expresses a reluctance torque generated by a difference between the inductance on the axis d and the inductance on the axis q. Refer to a treatise published in T. IEE Japan, Vol. 117-D, No. 8. 1997 for further detail.
In the rotor core 2 shown in FIG. 9 as another conventional example, a permanent magnet 6 having arch-shaped cross-section is used, and the torque T thereof is also found by the aforementioned operational expression.
Typically, ferrite magnet and rare-earth magnet are used for the permanent magnets 3 and 6 employed in the aforementioned type of the electric motor.
The ferrite magnet is less expensive and available for forming the permanent magnet in various shapes due to easiness of forming thereof, but the magnetic flux density is low, therefore hindering the reduction in size of the rotor core.
On the other hand, the rare-earth magnet has a high magnetic flux density, so that the reduction in size of the rotor core is easy, but the shape of the permanent magnet is limited by difficulty of forming thereof. In addition, the rare-earth magnet is more expensive than the ferrite magnet.
Since both the ferrite magnet and the rare earth magnet have merits and demerits as explained above, conventionally for reasons of the use and/or a cost of a motor, either the ferrite magnet or the rare-earth magnet is chosen for all permanent magnets of magnetic poles. In this case, there are disadvantages described below.
For example, in the case that the magnetic pole is formed of only the ferrite magnet, the amount of the magnet should be increased to increase the magnetic flux density. Therefore the polarized area becomes large, and consequently, the magnet occupies the most of the rotor core. Accordingly, the inductance on the axis q is small, so that the difference of the inductance on the axis q and the inductance on the axis d (parameter of a reluctance torque: refer to the aforementioned operational expression) becomes small, with the result that a sufficient reluctance torque cannot be attained.
In the case that the magnetic pole is formed of only the rare-earth magnet, the magnetic flux density is high, so that the magnet does not occupy the most of the rotor core as the ferrite magnet, but the magnetic flux density is often excessively high, and additionally the rare-earth magnet is expensive, therefore the motor is increased in cost.
As described hereinbefore, conventionally, a proper permanent magnet having an intermediate state between the ferrite magnet and the rare-earth magnet is troublesome to obtain, that is to say it is difficult to select the required magnetic flux density and reluctance torque, with low cost.